How to render the viewport with EEVEE and the "final" render with Cycles?
The viewport is chosen automatically based on which shading mode you are in no need to do any additional setup.

For LookDev, and Rendered modes it uses EEVEE is used automaticaly, for other modes the Workbench engine is used.

To pick your current render engine used for image output go to the Properties Window > Render, tab and pick your engine from the list.